This invention relates to the field of glazings and glazing systems, including windows, skylights, atriums, and greenhouses. More specifically, it relates to honeycomb transparent insulation materials that are used as components in glazing systems.
Honeycomb transparent insulation was first developed in the early 1960""s in order to enhance the insulation value of glazed systems, with minimum loss of light transmittance. Honeycomb transparent insulations are transparent-walled honeycombs, with open-ended cells whose axes are oriented parallel to the normal vector of the plane of the glazing. Honeycomb transparent insulation materials achieve high light transmittance because the cell walls are perpendicular to the plane of the glazing, and thus, any light that reflects from the cell wall continues in the forward direction. Thus these materials avoid the reflection-loss penalty that is incurred when extra glazings are inserted in the standard plane-parallel orientation.
Honeycomb transparent insulation materials provide insulation value by suppressing both convection and radiant heat. Honeycomb transparent insulation materials are typically made from transparent plastics such as acrylic, polycarbonate, or polypropylene. They are manufactured by a number of different techniques, including capillary bundling, extrusion, and film-fabrication. Their properties (such as light transmittance, insulation value, rigidity, weight, etc.) strongly depend on how they were manufactured. Examples of honeycomb transparent insulations are InsolCore(copyright), a film-based transparent insulation made by Advanced Glazings Ltd., Nova Scotia, Canada, Kapillux(copyright), a capillary-bundled transparent insulation made by Okalux Kapillarglas Gmbh. of Marktheidenfeld-Alffeld, Germany, and AREL(copyright), an extruded transparent insulation made by Arel Energy Ltd., Yavne, Israel. The mechanisms by which heat transfers through honeycomb transparent insulation materials are well understood. They are well-described in the technical literature (xe2x80x9cCoupled Radiative and conductive heat transfer across honeycomb panels and through single cellsxe2x80x9d, K. G. T. Hollands et al., Int. J. Heat Mass Transfer v.27, n.11 pp. 2119-2131, 1984, xe2x80x9cAn approximate equation for predicting the solar transmittance of transparent honeycombsxe2x80x9d, K. G. T. Hollands, K. N. Marshall, and R. K. Wedel, Solar Energy, v.21 pp. 231-236, 1978). Like many other thermal insulators, honeycomb transparent insulations work by dividing an air gap into spaces that are too small to support free convection. It has been found, both experimentally and theoretically, that honeycomb cells with a hydraulic diameter on the order of 1 cm are sufficiently small to suppress free convection. (xe2x80x9cDimensional relations for free convective heat transfer in flat-plate collectorsxe2x80x9d, K. G. T. Hollands, Proceeding of the 1978 Annual Meeting, ASES/ISES, Denver, Colo., vol. 2.1 pp 207-213, 1978). Thus an appropriately-designed honeycomb transparent insulation material does a thorough job of creating a dead air layer. Using smaller cells provides little improvement in suppressing the non-radiative portion of heat transfer, but does increase the amount of material required to manufacture that honeycomb transparent insulation.
To achieve maximum insulation value, a material must suppress radiative heat transfer in addition to conduction. The rate of radiant heat transfer through a honeycomb transparent insulation depends on the thermal-radiative emissivity of the boundary (i.e. the sheet(s) of glass or plastic adjacent to the honeycomb), the thermal-radiative emissivity of the cell wall, and the aspect ratio of the cell (defined as the ratio of the cell""s hydraulic diameter to its length) .
Boundary emissivity is generally a function of the glazing system in which a transparent insulation is used, and not a function of the transparent insulation material itself. Thus, for the sake of simplicity, the scope of background discussion will be limited to systems with high-emissivity boundaries, on the order of 0.9, such as are common for surfaces of glazing materials such glass or sheet plastics of thickness 0.030 or more. However the present invention can be used in glazing systems with other boundary emissivities.
To improve the radiant suppression (and therefore improve the insulation value) of a typical honeycomb transparent insulation made with plastic walls which are partially transparent to thermal radiation, it is necessary to do one of the following: (1) increase the aspect ration of the honeycomb; or (2) increase the emissivity of the cell walls. To accomplish option (1), it is necessary to either use a smaller cell diameter or a larger cell length (i.e. overall honeycomb thickness). But both of these modifications mean extra material usage and costxe2x80x94material content increases with the inverse square of the cell diameter, and in proportion to the honeycomb thickness. Also, practical limitations may discourage greater thickness: for example, finished glazing units made of such insulation sandwiched between glass may be too thick to work with existing framing systems. Thus, Option (2), increasing wall emissivity, is attractive, and forms the basis for the present invention.
For materials and geometries typically found in honeycomb transparent insulations, cell-wall emissivity is a function of wall thickness and the type of material from which the wall is constructed. Present honeycomb transparent insulation materials are almost exclusively made from plastics such as polypropylene, acrylic, and polycarbonate, with typical wall thicknesses of 0.001xe2x80x3 to 0.005xe2x80x3, and have non-optimal wall emissivities (typically 0.15 to 0.40). As a result, present honeycombs have non-optimal ratio of performance to material content. This situation could be remedied by simply increasing wall thicknesses, but this is undesirable because the material content increases, raising the cost and weight.
Thus it is highly desirable to use a material that is inherently a strong absorber of thermal-infrared radiation. Inorganic materials such as glass or silica are highly-attractive materials for making transparent honeycombs, having excellent clarity and high emissivity for small thicknesses (a layer of glass thickness of 0.0003xe2x80x3 has an emissivity of about 0.85). However, such materials are inherently difficult to work with in typical honeycomb geometries, and this has prevented the development of optimal glass honeycombs. Plastics, despite their imperfect radiative properties, are much easier to work with, and thus are the material of choice for today""s commercial honeycomb transparent insulation.
Composite materials made with finely-divided inorganic fillers in plastic resins are commonplace in today""s material technologies. Glass-filled thermoplastic resins are readily available to plastic processors, where typically the glass has been added to alter the elastic modulus or other physical properties of the plastic. Diatomaceous earth and calcium carbonate are regularly added to plastic resins when processing into plastic film in order to provide anti-block properties.
The addition of finely-divided inorganic fillers to plastic to create plastic film with enhanced infrared absorption is known. An example is xe2x80x98infrared-blockedxe2x80x99 polyethylene film for covering greenhouses, such as xe2x80x98Durathermxe2x80x99 (AT Plastics, Toronto, Canada). A number of additive concentrates are readily available for creating such films. An example is Ampacet additive concentrate Product 10021 B-U (Ampacet Corp, Tarrytown, N.Y.) which contains a high percentage of Kaolin, a fine white silicate clay that does not absorb visible light but effectively absorbs infrared radiation, in a linear low-density polyethylene/ethylene vinyl acetate carrier. The presence of Kaolin typically interferes with the passage of visible light by increasing scattering (i.e. haze). Any haze in the wall of a honeycomb transparent insulation material will reduce the light transmittance via backscattering, and this can be advantageous or problematic, depending on the intended application.
U.S. Pat. No. 5,256,473 describes a finely-divided silica which is made by a water-milling process, that can be blended into clear thermoplastic resin during film-making, in order to make a clear, high-silica-content composite film with enhanced thermal infrared absorption. The film was described as useful for agricultural (greenhouse) and packaging applications.
U.S. Pat. No. 5,683,501 describes formulation that consists of a high loading of ultra-fine silica particulate, along with a plastic resin, in a liquid dispersion. This formulation is intended to as a coating that dries to a thin film with high clarity. This formulation is said to have advantages when used as a clear protective film, with respect to increased hardness, weatherability, and durability. The patent makes no mention of increased thermal-infrared absorption, although such a formulation will inherently have high absorption because of the silica content.
GB patent no. 1,555,795 describes a solar collector including a sheet containing a dispersed material to enhance absorption of long-wave radiation. However, such a sheet would typically tend to be cloudy and not suited for use in transparent insulation application.
Various inorganic particles, powders, fibers, etc., have varying compatibility with plastic resins with respect to the amount of effort required to uniformly disperse the additive, and with respect to the alteration of the crystallization of the thermoplastic structure (altering the crystallization mechanisms can potentially cause optical inhomogeneities and haze). Inorganics such as metal oxides tend to be hydrophilic, while plastics tend to be hydrophobic, and typically, the inorganics will tend to clump together to form larger aggregate particles during processing. As well, the plastic does not effectively xe2x80x98wetxe2x80x99 to the surface of the inorganic particles, and the resultant composite can have small gaps between the plastic matrix and the inorganic particles. These gaps contribute to light scattering and haze, and this effect can increase if the composite is physically stressed.
The mechanics of dispersing inorganic particles in plastic resins is well know in the arts of plastics processing, paint making, and other areas. Before dispersion, inorganic particles are often pre-coated with materials that increase their dispersability or alter other properties. Examples of materials used to precoat inorganic particles are organo-silanes, stearates, heavy alcohols, anionic surfactants, and waxes. Such techniques are applicable to inorganic pigments as well as to non-colouring materials. A number of techniques for improving dispersion of inorganic particles are described in the patent literature. For example, U.S. Pat. No. 4,283,322 describes a binder composition for coating glass fibers to improve their compatibility when blended with polypropylene. U.S. Pat. Nos. 5,318,625 and 5,830,929 describe organic-based coating treatments for improving the dispersibility of inorganic pigments or fillers, with emphasis on titanium dioxide, a commonly-used white pigment. U.S. Pat. No. 3,992,558 describes a process by which inorganic particles can be coated with a thin layer of a thermoplastic prior to blending into a thermoplastic compound, and it is claimed that very high loadings of inorganics can be achieved in this way.
According to the present invention there is provided a clear honeycomb transparent insulation comprising an array of open-ended cells whose axes are oriented normal to the plane of the insulation, said cells having walls comprising a composite material made of inorganic particles dispersed in a plastic resin binder, said inorganic particles and said plastic resin having similar refractive indices, characterized in that the size of said organic particles lies in the range 1 to 10 xcexcm.
The basis for the present invention is the recognition that a composite material made from an inorganic, preferably of high-emissivity, and a plastic resin binder solves problems related to the use of either plastics or inorganics alone and can provide a clear material suitable for use as a transparent insulation.
An optimum material for manufacturing honeycombs combines the workability of plastics, with the strong thermal-radiative emissivity of inorganics such as glass, silica, alumina, silicate clays, or similar. Such a material can be created by dispersing a finely-divided inorganic particulate within a plastic resin which serves as a binder matrix. This inorganic-filled plastic can then be used to manufacture honeycomb transparent insulation, using techniques know to the the art, such as extrusion, capillary-bundling, and film-fabrication. A honeycomb transparent insulation made from such an inorganic/plastic composite represents an advance in the state of the art, because it provides better thermal insulation than an identical honeycomb transparent insulation made of plastic. This enhanced insulating capacity is a result of the additional radiative suppression provided by the presence of the inorganic particulate in the honeycomb cell walls. Also, inorganic/plastic composite-based honeycomb transparent insulations can be more stable and durable than plastic honeycombs, because the inorganics can provide increased resistance to UV and thermal degradation.
This invention can be used to create honeycomb transparent insulations that are dimensionally similar to those typical of the present state-of-the-art, yet offer increased insulation value. Alternatively, the invention can be used to create honeycombs that have thinner walls and/or lower aspect ratio, yet have insulating capability similar to state-of-the-art honeycombs. These thinner-walled and/lower aspect ratio honeycombs contain less material and are therefore lighter and more economical.
This invention may be implemented using a clear composite formulation, resulting in a honeycomb transparent insulation with maximum light transmittance. Additionally, this invention may be implemented with hazy or diffuse composite formulations, resulting in a honeycomb transparent insulation with reduced transmittance through backscattering losses. This can be used advantageously in systems for applications such as daylighting, where reduced light transmittance is desired. By reducing transmittance through backscattering, the glazing system avoids the internal heat buildup that would result if an absorption-based attenuation scheme was used.
The invention also provides a method of making honeycomb insulation comprising the step of fabricating honeycomb cells from a material including an inorganic/plastic composite to increase suppression of radiant transfer.